Suitability of Welsh onion and broccoli cultivation in a heavy clay field equipped with a subirrigation and drainage system

Incorporating vegetable cropping into a lowland crop rotation system (i.e., rotation of irrigated rice and upland crops) could be economically beneficial for farmers. However, upland crops grown in the heavy clay paddy fields typical of the Hokuriku region of Japan would suffer soil moisture stress due to both too much and too little water. The Farm-Oriented Enhancing Aquatic System (FOEAS) is a new subirrigation and subdrainage system designed to improve the suitability of converted fields for upland crops by preventing extremes of soil water. We grew Welsh onion (Allium fistulosum L.) and broccoli (Brassica oleracea L. Italica Group) in a heavy clay converted field equipped with FOEAS at the Hokuriku Research Center, and monitored crop growth, soil water content, and water table depth in 2011 and 2012 (one cultivation period for each crop per year). In one treatment plot, subirrigation was used to maintain the water table at 30 to 40 cm below the soil surface (subirrigation, SI plot). In the other, the water table depth was not controlled (no irrigation, NI plot). Each plot was divided into two fertilizer application subplots in 2011. The conventional application subplot received a basal application of fertilizer (N:P2O5:K2O = 5.2:5.2:5.2 g /square m for Welsh onion and 22.6:21.0:20.6 g /square m for broccoli) which were mixed in the whole plow layer through plowing process (the depth of plowed layer was about 20 cm) before planting and followup topdressings (N:P2O5:K2O = 9.0:8.7:7.5 g /square m for Welsh onion and 7.2:0.0:7.2 g /square m for broccoli). The band application subplot received only a basal application of fertilizer (N:P2O5:K2O = 11.4:11.1:10.2 g /square m for Welsh onion and 23.8:16.8:22.2 g /square m for broccoli) including controlled-release urea in a subsurface band 10 cm deep along the seedling row after plowing and before planting. In addition, we tested the effect of postplanting subirrigation on the rooting and growth of broccoli in summer in 2012. The results are summarized as follows: 1) The water table depth could not be maintained by the subirrigation supply at the set value in the SI plot throughout the growing period, probably because of the low permeability of the soil and the slow capillary rise. Nevertheless, soil moisture in the root zone did not fall below -100 kPa in either plot, and both crops grew generally well except for following cases. 2) Deterioration of the subdrainage performance occurred in the SI plot during growing period of broccoli in 2011, probably due to trying to keep relatively high water table depth (-30 cm) under the situation of higher frequency of precipitation. As a result, soil moisture in the root zone frequently rose too high (>-1 kPa) and the growth of broccoli was inferior in the SI plot compared to the NI plot. 3) Subirrigation immediately after planting of broccoli in summer in 2012 raised the water table depth temporarily to near the surface and provided adequate soil water in the SI plot, enabling good rooting and better initial growth than in the NI plot. Crop growth was more advanced and the total harvestable yield was higher in the SI plot. Although sprinkling irrigation via a perforated tube after planting in the NI plot also enabled good rooting, the water quantity was too small to raise the soil water content sufficiently for good initial growth comparative to SI plot. 4) The growth of Welsh onion and broccoli in the band application subplot was at least as good as that in the conventional application subplot, even though the band application subplot received only 80% of the fertilizer applied to the conventional subplot. Thus, this study demonstrated that Welsh onion and broccoli grew generally well in a heavy clay converted field equipped with FOEAS. The expansion of Welsh onion cultivation in such fields would need the development of farm machinery adapted to the heavy soils. Although yield increases by continuous subirrigation might be limited, our result suggests that postplanting subirrigation could lead to good growth and yield of broccoli. Broccoli cultivation in heavy clay converted fields could become increasingly practicable because costs and labor for postplanting irrigation could be saved by using FOEAS subirrigation.